Archibald Alexander: Christ satisfied all requirements that the first Adam failed to perform

Archibald Alexander explains, in terms of the covenant of works, that Christ satisfied the penalty and purchased the reward of eternal life for all His elect, given that paying the penalty did not do away with the preceptive requirements of perfect holiness with respect to the law:

Suppose that the first Adam had continued to obey until his probation was finished, would any one think that afterwards either he or his posterity would be freed from the obligation to be holy? Well, what he failed to do, the second Adam has performed, but the obligation to be holy is immutable. It may be asked, does the law of God require a double obedience, one from our surety, and one from ourselves? We answer, that it requires but one righteousness in order to our justification; but it requires that the justified person continue in conformity with its holy precepts. Our obedience is not now required as a condition of justification; to entertain such an opinion would be to leave the covenant of grace, and to go back to the old covenant of works. It would be to fall from grace, as Paul expresses it, that is from the doctrines of grace. Suppose each one of us had a probation for life under the law, and that we had completed our obedience and obtained justification, we should be required to render no more obedience with a view to being justified, for this is supposed to be already done. But the obligation to obey God would not cease, because we were in a justified state. We would still be required to be conformed to the law, because that was our reasonable service, arising out of our natural relations to our Creator, and because holiness is pleasing to God, beneficial to men, and essential to the promotion of our own happiness. (Archibald Alexander, A Treatise on Justification by Faith)